Silicon wafers useful in the manufacture of semiconductor devices require close scrutiny to detect defects as soon as possible in the manufacturing process. Several apparatus are known in the art for detecting microscopic defects on the surface or near the surface of such devices. One such apparatus utilizes a laser beam that is scanned over the surface of a wafer and includes means for detecting scattered radiation from the wafer surface. The specular reflection is blocked from the detection device by suitable arrangement of the lenses and spatial filters. If the surface of the wafer has an imperfection such as dirt, hills, scratches and the like, the laser beam will be scattered from the imperfection. There are also scattering processes such as Raman scattering, etc., which occur, but the intensity of the light due to such scattering effects is usually negligible. The scattered light from the wafer is collected from about the main axis of the lens and is focused on a detector. The scattered light is converted to electrical impulses which can be counted or, in the alternative, can be displayed as a bright spot on an oscilloscope or other monitor. See U.S. Pat. No. 4,314,763 issued Feb. 9, 1982 to E. F. Steigmeier et al. entitled "DEFECT DETECTION SYSTEM" for a detailed description of such a scanning apparatus.
The use of such light scattering apparatus for detecting surface and subsurface defects by conventional light scanning techniques does not identify or test for the crystalline quality of the semiconductor material. The quality of such material is related to the purity or perfection of the crystallographic growth of the material on an atomic or microscopic scale. Deviations from the ideal crystallographic perfection can be said to be a reduction in the quality of the material. The better the quality the closer the material is to the ideal crystallographic perfection. The term "crystallographic quality" includes structural conditions known to more or less extent in the art. For example, "mosaic spread" can be a deviation from the ideal crystallographic structure caused by slight misorientation of the crystalline axis directions or mosaic spread can be manifested by larger misorientations of one area of the material against the adjacent area. Such larger misorientations might be called "grain" or "twinning." Other structural deviations of the crystalline structure of a semiconductor material are continuously being identified and analyzed in the art. However defined, the quality of the crystalline structure, it should be understood, is distinguished from the defects on the surface of the semiconductor material in the form of scratches, recesses, particulates, and the like.
In U.S. Pat. Nos. 4,352,016 and 4,352,017, issued Sept. 28, 1982, both entitled A METHOD AND APPARATUS FOR DETERMINING THE QUALITY OF A SEMICONDUCTOR SURFACE, based on the inventions of M. T. Duffy, P. J. Zanzucchi, and J. F. Corboy, Jr., there is described a method and apparatus for determining the quality of the material of a semiconductor surface. In brief, the surface quality of the semiconductor material is determined by exposing the semiconductor surface to two light beams of different wavelengths or wavelength ranges (e.g., ultraviolet at 2,800 angstroms and near ultraviolet at 4,000 angstroms). A portion of each of the respective light beams is reflected from the semiconductor surface. The intensity of each reflected beam is measured to obtain an intensity difference whereby the magnitude of the difference is a measure of the quality of a semiconductor material. While the quality of semiconductor material can be tested or evaluated quite well using the ultraviolet two wavelength technique described in the above-identified Duffy, et al. patents, the time required to make such tests can be very long and not well suited for on-line evaluations needed in modern day semiconductor processing and manufacturing.